1. Field of the Invention
The invention relates to a motor improved in acceleration/deceleration performance and, more particularly, to a motor improved in acceleration/deceleration performance as a motor for use in peripheral equipment of computers such as a spindle motor for an optical disk.
2. Description of the Prior Art
Recently there have been made remarkable advances in technologies for attaining higher performance, especially higher speed operation, of various types of OA machines. Accordingly, there are demands for higher revolving speeds achieving higher speed operation of spindle motors for driving disks in various machines. In the case, particularly, of a spindle motor for the CD-ROM under the CLV (Constant Linear Velocity) control to keep the linear velocity constant, it is demanded of the motor to keep the accelerating/decelerating time within the limit of a required length of time, as well as to rotate at high speeds. These motors are generally the outer rotor type such that as large a torque as possible is obtained. However, such a system, although torque constant indicating the motor performance can be easily obtained therein, is not the best one, because in achieving higher speed operation accompanying a large number of revolutions, the torque constant must be kept low.
A problem involved in the achievement of higher speed operation is the characteristic of the accelerating/decelerating time. In the CD-ROM drive, since it drives a disk of a diameter of 12 cm, it must provide a large moment of inertia and, hence, it especially is difficult to keep the decelerating time within specified limits and this has been a big problem. Therefore, in the products of an octuple speed or higher multiple speed, the CAV (Constant Angular Velocity) control is used jointly with the CLV control to widen the range in which the number of revolutions is kept constant so that the variation in the number of revolutions is reduced. Consequently, it becomes necessary to change the read frequency between inner and outer circumferential portions and, hence, some control is required to be made on the side of the read device. Further, since the rated rotating speed represents the average speed or the maximum speed, it is becoming impossible to imply xe2x80x9cconstant linear velocity at any positionxe2x80x9d as with the case of lower multiple speed products. Further, by lowering the torque constant for achieving higher revolving speeds, the power consumption is much increased under the same load and, hence, the effect of heat on the optical head or the like produces problems and, also from the point of view of energy saving, it is required to improve the efficiency.
FIG. 10 is a structural drawing of an outer rotor type DC brushless motor of the prior art, in which reference numeral 101 denotes a lead wire formed of a flat cable serving as the interface of the motor. Reference numeral 102 denotes a circuit board on which the body of the motor is installed and control elements, wiring circuits, and the like are disposed. Reference numeral 103 denotes a driving coil. Reference numeral 105 denotes a stator yoke built up from laminations of soft magnetic sheet steel and it has salient poles provided on the outer circumference. Reference numeral 123 denotes a rotor. The rotor 123 includes a shaft 106, a turntable 107, a rubber 108, a magnet 109, back yokes 113 and 118, and a rotor magnet 115. Reference numeral 112 denotes a substrate for mounting parts and the like thereon, 119 denotes a bearing, 120 denotes a position detecting sensor, and 122 denotes a position detecting switch.
In the prior art outer rotor type DC brushless motor, while nine driving coils 103 are disposed centering around the shaft 106 and evenly distributed along the circumference of it, the driving coil 103 cannot be disposed beyond the position of the rotor magnet 115 because the rotor 123 is of the outer rotor type. Accordingly, it is impossible to obtain a sufficient coil space to place therein a decelerating coil for braking. Nevertheless, there is left a wasteful space on the side opposite to the optical head, i.e., on the substrate indicated by character A.
The invention was made in view of the above described problems. Accordingly, it is an object of the invention to provide a motor, or more particularly a high speed motor for driving optical disks such as CD-ROM, CD-R, and DVD-ROM, improved in the accelerating/decelerating characteristics and able to keep the current consumption as small as possible.
In order to achieve the above described object, the invention provides a motor structure of an inner rotor type comprising a rotor magnetic pole made of permanent magnets forming magnetic poles on the outer periphery and rotating centering around the shaft and a stator magnetic pole having salient poles with a driving coil wound around the same and disposed in confronting relationship with the rotor magnetic poles, the improvement comprising a plurality of stator magnetic poles projecting inwardly from the stator yoke surrounding the outer periphery of the shaft, of which plurality of stator magnetic poles, some, at least the same number as the number of the phases of the driving voltage, are each provided with a driving coil and a decelerating coil wound around the same and form stator magnectic poles having short salient poles, while the remaining stator magnetic poles are each made shorter than the long stator magnectic pole, provided with no winding, and form stator magnectic poles having short salient poles, and the rotor magnet being made up of permanent magnets disposed in confronting relationship with the salient poles of the stator magnetic poles for rotating centering around the shaft. Further, in configuring the motor in the invention, the inner rotor type was adopted to avoid its mechanical interference (contact) with the optical head fixed in place together with the motor and to obtain as large a coil space as possible, the driving coils and the decelerating coils are disposed on the side opposite to the position where the optical head is disposed, and the range within which they are disposed is set to 200 degrees or less. Hence, the shape of the stator is made smaller on the side toward the optical head and larger on the opposite side. Further, as a matter of course, there is provided some relief at the portion where it is required for installing the motor and, hence, the exterior view of the stator yoke assumes an odd shape having protrusions and depressions so that as large a space as possible is available for placing coils therein. While, as the main design, the decelerating coil for deceleration only is disposed on the inner circumferential side of the salient pole opposing the rotor magnet across a minute space, i.e., on the side toward the rotor magnet, and the driving coil is disposed on the outer side, the decelerating coil and the driving coil may be disposed at the same position or they may be placed at the same position but one in the inner layers and the other in the outer layers. Here, the decelerating coils are each provided for each phase and they are connected in a star or delta network and provided with their operating switches provided at the terminals of the coils. Further, the number of turns of the decelerating coil is made larger than that of the driving coil, so that the inconsistent requirements, i.e., to lower the torque constant (Kt) as the condition for high speed rotation of the motor and to increase the induced electromotive force as the condition for shortening the decelerating time, are met. Thereby, the decelerating coil becomes more effective in achieving the deceleration within a minimum amount of time. Further, on the circuit board, which also serves as the relays of the electric circuits of the coils, there are disposed transistors serving as the switches for the decelerating coils. Thus, by having the circuits for deceleration taken in the motor side, it was arranged such that the motor seen from the controlling side is virtually not different from the product of the prior art. Thereby, it has been made possible to obtain, from the motor size virtually not different from the prior art product, a larger number of revolutions and a shorter decelerating time at the same time. Further, as for the cogging as a problem with the inner rotor type, the salient poles of the short stator magnectic poles are evenly disposed on the plane opposing the salient poles of the long stator magnectic poles, as with the prior art inner rotor type, so that the magnetism in the rotor is balanced and the cogging is suppressed. Hence, while it is a motor of the inner rotor type, the cogging therein is kept at the same level as that in the outer rotor type of the prior art. Further, since the inner rotor type generally has smaller rotor diameter and smaller magnetic pole pitch, a sine waveform of the magnetization waveform can be kept up even if the number of poles is small and higher harmonic components adversely affecting the characteristics can be reduced. Therefore, the driving frequency can be set low without deteriorating the characteristics. By the decrease in the driving frequency, the eddy current loss caused by the alternating magnetic field could be reduced, the current consumption kept low, and the motor efficiency improved.
By means of the above described configuration, it becomes possible to provide a high efficiency motor capable of achieving at the same time both higher speed operation and reduction in accelerating/decelerating time.